In printing a source image created by using process colors of C (Cyan), M (Magenta), Y (Yellow) and K (Key Plate) on a transparent substrate or a colored opaque substrate, a technique to print a base or undercoat colored in W (White) (hereinafter, referred to as a white base) together with the source image is used in view of the fact that the appearance of the printed source image can change in color by being affected by light coming through the transparent substrate or the color of the opaque substrate. In printing a source image on a transparent substrate, a white base is generally printed as the uppermost layer on the substrate, because the printed source image is shown from the opposite side of the printed surface of the transparent substrate. In printing a source image on an opaque substrate, a white base is generally printed as the lowermost layer on the substrate.
In printing a source image with a white base on a transparent substrate or an opaque substrate, it takes time to print the source image and the white base separately, and thus there are demands for on-demand printers with high productivity, capable of printing five colors of W for a white base and CMYK for a source image in a single pass. When executing the above printing by using electrophotography, there is a limit of the amount of color materials (the amount of toners) that can be transferred and fixed on a substrate at one time, and therefore, a color material limit, which is generally given by the total sum of pixel values (in percentages) corresponding to all the colors of color materials, in other words, the total sum of percentage values of all the colors of color materials specified for a pixel, is defined for such printing. In view of that, a printing system is configured to execute a process of controlling or limiting the amount of color materials, which controls pixel values of a white base and a source image so that the total sum of the pixel values corresponding to the color materials of the white base and the source image at the same printing position does not exceed the color material limit.
As an example of the technique of controlling the amount of color materials, Japanese Unexamined Patent Publication (JP-A) No. 2009-029079, which corresponds to United States Patent Application Publication No. US2009/033956A1, discloses the following image processing method applicable to an image output device. The image output device is configured to use a base color material and process color materials to form a color image on a sheet, wherein colors of the image are defined on the basis of the total amount of the color materials given by a combination of the color materials per pixel. The image processing method includes a process of determining the total amount of the color materials given by a combination of the base color material and the process color materials per pixel, on the basis of an input pixel signal. The image processing method further includes a process of adjusting the number of the color materials to be combined, which includes a process of, if the determined total amount of the color materials is of a value in excess of an upper limit of the total amount of the color materials given by a combination of the color materials per pixel, which is defined for the image output device, reducing the number of the base color material or process color materials to be combined while maintaining colors to be produced, so that the total amount of the color materials does not exceed the upper limit of the total amount of the color materials.
As another example of the technique, JP-A No. 2009-055600, which corresponds to United States Patent Application Publication No. US2009/033961A1, discloses the following image processing method applicable to an image output device. The image output device is configured to use a base color material and process color materials to form a color image on a substrate, wherein colors of the image are defined on the basis of the total amount of the color materials given by a combination of the color materials per unit area. The image processing method includes a process of determining the provisional total amount of the color materials by adding the amount of the base color material being a certain value to the amount of the process color materials per unit area which are represented by a device signal supplied to the image output device. The processing method further includes a process of adjusting the total amount of the color materials which includes a process of reducing the provisional total amount of the color materials so that the provisional total amount of the color materials does not exceed the upper limit of the total amount of the color materials, defined for the image output device.
From an idealistic viewpoint, in order to secure the sufficient color reproducibility of a source image in printing the source image with a white base, the color material limit should be given by the sum of the maximum total amount of color materials of CMYK used in the source image and 100%. However, the color material limit that normal printing devices can handle is 400% or less, and it would be difficult to increase the color material limit because it needs a significant improvement of the capacities of the printing devices for transferring toner and fixing toner on a substrate and a significant improvement of color materials. One of actual solutions for the problem is a reduction of the amount of color materials, but a reduction of only either of the color materials of the source image or the color material of the white base has adverse effects on a color reproduction of the source image. Consideration is given to the reduction of the amount of color materials, with reference to the example that a printing device outputs secondary colors which are frequently used in color printing, wherein the color material limit of the printing device is less than 300%. The secondary colors are colors represented by combinations of the maximum possible values of two of the CMY primary colors specified for a pixel (the maximum possible pixel values indicating two of the CMY primary colors), such as R (Red) being a mixture of 100% M and 100%Y, G (Green) being a mixture of 100% C and 100% Y, and B (Blue) being a mixture of 100% C and 100% M, and are hereinafter referred to as pure secondary colors. In this example, both of the amount of color materials of the primary colors (C, M, Y) to be used for the source image and the amount of the white color material to be used for the white base should be reduced.
Consideration is further given to the reduction of the color materials to be used for the source image and the white base, with reference to two-valued pixels given after a screening (halftoning) process by using area gradation modulation or area coverage modulation. A screening (halftoning) process makes two-valued pixels including “colored” pixels and “uncolored” pixels for each of colors including the primary colors and white. Since a general screening process defines the arrangement of “colored” pixels and “uncolored” pixels in a stochastic manner, a composite image made of two-valued pixels corresponding to the primary colors and white given after the screening process, includes pixels colored in both of the primary color(s) and white, pixels colored just in the primary color(s), pixels colored just in white, and pixels colored in none of the primary colors and white. After studying about these types of pixel, the inventor has found that, if a source image has high-chroma color (color close to one of the pure secondary colors), pixels colored just in white excluding the primary colors have adverse effects on reproduction of the color (contribute less to realize high chroma), and if the source image has high-lightness color (color close to white), pixels colored just in white excluding the primary colors have favorable effects on reproduction of the color (contribute to realize high lightness).